The present invention is directed to high green strength reactive hot melt adhesives prepared from, in the first step of the reaction, a high molecular weight hydroxy terminated polyester prepolymer which will crystallize at a useful temperature and generate tough, fast setting properties in the uncured adhesive.
Reactive hot melts are one-component, 100% solids, solvent-free urethane prepolymers. Unlike conventional hot melts that can be repeatedly heated from its solid state and flowed to a liquid form, the reactive hot melt behaves as a thermoset and goes through an irreversible chemical reaction once dispensed in the presence of ambient moisture.
The reactive hot melts are isocyanate terminated prepolymers that react with surface or ambient moisture in order to chain-extend, forming a new polyurethane polymer which offers performance superior to that obtained with conventional hot melts.
These reactive hot melts will bond to a variety of substrates including plastics, woods, fabrics and some metals making them ideal candidates for bonding dissimilar substrates. Moreover, they are flexible and durable by nature, so they may be used in extreme temperature ranges of xe2x88x9230 to +150xc2x0 C. while providing excellent moisture and chemical resistance.
High molecular weight polymers, typically polycaprolactone, HO[(CH2)5OCO]nH, or copolymers containing polycaprolactone, can be added as preformed polymers in a granular state to low molecular weight polyesters, polyethers or mixture of polyesters and polyethers prior to reaction with a diisocyanate to form a polyurethane hot melt adhesive. The advantage of adding a high molecular weight polymer with crystallizing properties is the generation of high uncured green strength. When a crystalline polymer is included, the crystallization is usually suppressed by reduced polymer mobility in the matrix. For these reasons, polycaprolactone, a polymer which retains its crystalline properties, is the best choice. However the high molecular weight needed to obtain useful properties results in a very high viscosity polymer which can only be practically used in granular form.